Among induction cooking stoves that produce high-frequency magnetic field with an induction heating coil for heating an object to be heated such as a pan with eddy current generated by the electromagnetic induction, there have been proposed certain types that can heat objects made of aluminum.
FIG. 4 is a cross sectional view of a conventional induction cooking stove. Top plate 2 is mounted to an upper part of main body 1 that composes an enclosure of the induction cooking stove. Top plate 2 is constructed of an insulating material such as ceramic and crystallized glass having a thickness of 4 mm, for instance. Utensil 3 to be heated such as a pan is placed on top plate 2. Induction heating unit 5 having heating coil (hereinafter referred to as “coil”) 4 is provided underneath top plate 2. Driving circuit 6 including an inverter supplies a high-frequency current to coil 4, which in turn generates high-frequency magnetic field to heat utensil 3 by magnetic induction.
In the conventional induction cooking stove of this type, an interaction between an electric current induced in the bottom of utensil 3 and the magnetic field generated by coil 4 produces a repulsive force on the bottom of utensil 3 in a direction of pushing utensil 3 away from coil 4. This repulsive force is comparatively small when utensil 3 is made of a material of high magnetic permeability and relatively large specific resistance such as iron, since it requires a small electric current to obtain the desired output of heating power. In addition, utensil 3 made of iron and the like does not move upward or sideways since it receives a magneto-attractive force as it absorbs the magnetic flux.
On the other hand, if utensil 3 to be heated is made of a material of high conductivity and low magnetic permeability such as aluminum and copper, coil 4 requires a large current to induce a large current in the bottom of utensil 3 in order to obtain the desired output of heating power. Consequently, this produces a large repulsive force. In addition, utensil 3 made of aluminum does not receive as large a magneto-attractive force as in the case of the material of high magnetic permeability such as iron. As a result, an interaction between the magnetic field of coil 4 and another magnetic field generated by an induced current in utensil 3 produces a large force in the direction of pushing the utensil 3 away from coil 4. This force acts upon utensil 3 as a lifting force. There is a possibility that this force lifts and moves the utensil 3 on a cooking surface of top plate 2 if the utensil 3 is not heavy enough. A phenomenon of this kind tends to occur rather notably when utensil 3 is made of aluminum of which a specific gravity is smaller than copper.
FIG. 5A is a schematic illustration showing a direction of electric current 4A flowing in coil 4, as observed from the side of utensil 3, and FIG. 5B is a schematic illustration showing a direction of eddy current 3A induced in utensil 3 by the electric current that flows in coil 4, as observed from the same direction as that of FIG. 5A. Eddy current 3A flows generally in the same circular pattern as electric current 4A of coil 4, but in the opposite direction, as shown in FIG. 5A and FIG. 5B. Therefore, these two circularly flowing currents resemble a pair of magnets having substantially same sectional area as the size of coil 4, disposed in a manner that same magnetic poles confront each other, namely N-pole against N-pole, for instance. As a result, utensil 3 and coil 4 produce a large repulsive force between them.
This phenomenon is very noticeable when utensil 3 is made of a material of high specific conductivity such as aluminum and copper. On the other hand, a utensil made of non-magnetic stainless steel generates a sufficient amount of heat even when the electric current supplied to coil 4 is small, because a specific conductivity of stainless steel is lower than aluminum and copper although it is a material of similarly low magnetic permeability. For this reason, coil 4 generates a weak magnetic field and induces a small eddy current to flow in utensil 3, thereby exerting a small lifting force on utensil 3 being heated.
As described above, there is the possibility that utensil 3 made of aluminum floats in the air and it is not heated properly due to the lifting force exerted on utensil 3 when used for cooking on the induction cooking stove. As a measure to resolve this phenomenon, Japanese Patent Unexamined Publication, No. 2003-264054 discloses a structure in which electric conductor 7 is provided between coil 4 and top plate 2 in a manner to be in close contact to top plate 2, as shown in FIG. 4. In this structure, magnetic field generated by coil 4 crosses both electric conductor 7 and utensil 3, and produces an induction current in both of them. In this case, an interaction between magnetic field generated by the induction current induced in electric conductor 7 and magnetic field generated by the induction current induced in utensil 3 converges the magnetic flux of coil 4 into the center area, which increases an equivalent series resistance of coil 4. This increase in the equivalent series resistance means a strong magnetic coupling between utensil 3 and coil 4. When the magnetic coupling becomes strong, coil 4 can generate an equivalent amount of heat in utensil 3 with a small electric current, and decrease the lifting force. This effect of decreasing the lifting force becomes greater the more the equivalent series resistance of coil 4 is increased by expanding a surface area of electric conductor 7 confronting coil 4. Here, the equivalent series resistance is defined as an equivalent series resistance in an input impedance of coil 4 as measured with a frequency approximating the heating frequency under the condition in which utensil 3 and electric conductor 7 are arranged in the same manner as the normal heating operation.
Since the adoption of electric conductor 7 decreases the lifting force as described above, it makes cooking practically possible by induction-heating utensil 3 made of a material having a high electric conductivity and low magnetic permeability such as aluminum.
However, it is necessary to control a total weight of utensil 3, or the pan, and food material so that they become heavier than a prescribed weight because the floating phenomenon of utensil 3 can not be completely disregarded in the actual use.
To solve this problem, it is considered practical to reduce the lifting force exerted on utensil 3 by increasing the surface area of electric conductor 7. In other words, there is the need to increase the equivalent series resistance of coil 4. To be specific, it is considered effective to reduce the aperture in the center of electric conductor 7 confronting coil 4 to such a dimension that leaves only a space necessary for temperature detector 8 mounted to top plate 2 for detection of its temperature. This can thus increase the surface area of electric conductor 7 and reduce the lifting force.
On the other hand, the reality is that not many pans have perfectly flat bottoms, but they normally have slightly warped bottoms. That is, the majority of pans used are inwardly warped in the bottom into a concaved shape.
However, when any of such warped pans is used for heating on the induction stove provided with electric conductor 7, the bottom of the pan stays far from coil 4. This decreases an amount of magnetic flux crossing the pan in an area corresponding to the center of coil 4, and increases the magnetic flux that crosses electric conductor 7, thereby resulting in an increase in the amount of heat generated in the inner part of electric conductor 7. This gives an extraordinary rapid rise in temperature of electric conductor 7 in an area near the center thereof. In addition, the heat generated in electric conductor 7 is prevented from being conducted to the bottom of the pan due to a void space between the warped portion of the pan bottom and top plate 2, and this further accelerates the temperature rise of electric conductor 7. It is also necessary to reduce an output power of coil 4 when the temperature of electric conductor 7 becomes too high, in order to suppress the heating of electric conductor 7 and to prevent the high temperature of electric conductor 7 from causing an adverse influence to coil 4 and the like components. This can be achieved by means of monitoring the temperature of electric conductor 7, for instance, so as to interrupt or regulate the heating output when the monitored temperature becomes too high. As a result, there may be a case that it takes too much time for cooking, or the cooking is not completed because the output of coil 4 is reduced prematurely if the temperature of electric conductor 7 rises so rapidly. For this reason, electric conductor 7 must be provided with a void area of a predetermined diameter in the center thereof, and this makes it difficult to decrease the lifting force.
There are also other kinds of electric conductors similar to the invention of this application, such as those disclosed in Japanese Patent Unexamined Publications, Nos. H07-249480, H07-211443 and H07-211444. However, none of the induction heating apparatuses disclosed in these inventions is provided with a heating coil capable of heating utensils made of aluminum, copper and the like materials having generally equivalent or higher specific conductivities as those. In other words, the electric conductors disclosed in these patent publications hardly show any effect of decreasing the lifting force when induction-heating utensils made of materials having comparatively high specific resistances such as magnetic iron and stainless steel.